1. Field of the Invention
This invention relates to an antenna assembly, and more particularly to a retractable antenna assembly suitable for use in a compact portable radio apparatus, such as, a cellular phone.
2. Description of the Prior Art
In recent years, portable radio apparatuses have been reduced in size and weight. Accordingly, antenna assemblies used for such portable radio apparatuses are also required to be smaller. As an antenna assembly which more or less satisfies the above requirement, many manufacturers are developing whip antennas which can be retracted into the housing when the apparatus is not used for communications. The whip antenna is pulled out of the housing to be extending therefrom when the apparatus is used for communications. Portable radio apparatuses in an early stage have utilized this type of whip antenna which is relatively simple.
Such a simple whip antenna, when extending from the housing body, is operative as a monopole antenna. However, when the antenna is in a retracted position within the housing, it cannot obtain a sufficient gain because the antenna retracted into the housing is placed near the ground where it causes an input impedance to increase, whereby impedance matching cannot be readily established.
In order to improve the gain of the antenna when retracted into the housing, a whip antenna of a so-called top loading type came into use instead of the simple whip antenna described above. The top loading type whip antenna comprises a helical antenna electrically connected to the top or free end of a rod antenna. When this type of whip antenna extends from the housing for use, radio waves can be radiated from both the helical antenna and the rod antenna. When the antenna is retracted into the housing, radio waves can still be radiated from the helical antenna.
However, in this type of whip antenna, the rod antenna does not contribute to the radiation of radio waves when it is retracted, but rather operates as an open stub which affects the input impedance of the antenna. Such open stub particularly causes disturbance of impedance matching to an extent depending upon the distance between the retracted rod antenna and circuit boards in the housing. Further, if a shield in the housing is not complete, signals are undesirably input through the rod antenna in a retracted position and such signals can penetrate to the inside of the shield.
Because of these problems associated with the whip antenna, another type of antenna assembly has been developed which includes a rod antenna and a helical antenna, but in which the rod antenna, when retracted into the housing, is electrically isolated from the helical antenna. An example of such a retractable antenna assembly will be described with reference to FIGS. 1A and 1B which show a conventional retractable antenna assembly 100 in an extended position and in a retracted position, respectively.
More particularly, the retractable antenna assembly 100 is shown to generally comprise a fixed antenna part 101, constituted by a one-piece helical antenna 102 housed within a cylindrical antenna cover 103, and a movable antenna part 104 constituted by a rod antenna 105 enveloped in a cover 106.
The fixed antenna part 101 further includes a metal end piece 107 in the form of a disk with an externally threaded hub 108 extending downwardly from the disk 107 about an axial bore 109. An annular metal plate 110 at the lower end of the helical antenna 102 electrically connects the latter to the disk or end piece 107 and the lower end of the antenna cover 103 is suitably secured on the periphery of disk 107 for positioning the cover 103 in enveloping relation to the helical antenna 102. The cover 103 has an upper end wall 111 above the upper end of the helical antenna 102 with a central bore 112 extending through end wall 111 in axial alignment with the bore 109 in hub 108. Such bores 109 and 112 are dimensioned so that the movable antenna part 104, that is, the rod antenna 105 and the associated cover 106, can move slidably through bores 109 and 112 between the extended and retracted positions of FIGS. 1A and 1B, respectively.
A knob 113 is formed on, or otherwise secured to, the upper end of cover 106 and may be grasped by the user for effecting movements of the movable antenna part 104 between its extended and retracted positions. The knob 113 is also effective to prevent inadvertent downward separation of the movable antenna part 104 from the fixed antenna part 101. An annular protrusion 114 is formed on antenna cover 106 immediately below knob 113 and is dimensioned for snap-in engagement in a similarly shaped indentation or detent formed in the bore 112 extending through end wall 111 of the cover 103. The engagement of the protrusion 114 in the detent or indentation in bore 112, as in FIG. 1B, is effective to releasably retain the movable antenna part 104 in its retracted position.
A stopper 115 is provided on the lower end portion of the cover 106 and is formed with an annular protrusion 116 which, upon movement of the movable antenna part 104 to the extended position shown in FIG. 1A engages, in a snap-in manner, in a similarly shaped indentation or detent 117 formed in the surface of bore 109. Such engagement of protrusion 116 within indentation 117 is effective to releasably retain the movable antenna part 104 in its extended position. In the illustrated antenna assembly 100 according to the prior art, the stopper 115 at the lower end of the movable antenna part 104 is of an electrical insulating material, for example, of the same material as the cover 106, so that an electrical connection to the rod antenna 105 cannot be established through the stopper 115.
In order to provide for the mounting of the retractable antenna assembly 100 on a cellular phone or other portable radio apparatus 120, the housing 121 of such apparatus is conventionally molded of a suitable plastic with an opening in which a conductive metal mounting member 122 is embedded. Such mounting member 122 has a boss with an internally threaded bore 123 in which the externally threaded hub 108 can be threadably engaged. When the antenna assembly 100 is thus mounted on housing 121, the helical antenna 102 is continuously powered from a power supply circuit 124 through an antenna clip 125 which establishes an electrical connection between a signal line extending from the power supply circuit 124 and the metal mounting member 122 which is, in turn, electrically contacted with the hub 108 of the disk 107, and through the annular metal plate 110 in electrical connection with the helical antenna .102.
When the movable antenna part 104 of antenna assembly 100 is in its extended position shown on FIG. 1A, rod antenna 105 extends through helical antenna 102. By reason of the foregoing, even though, in the extended position, stopper 115 of an insulating material insures that there will be no electrical connection between helical antenna 102 and rod antenna 105, the rod antenna 105 is powered by electromagnetic coupling between helical antenna 102 and rod antenna 105 extending axially therethrough. As a result, in the extended position, rod antenna 105 mainly operates as a monopole antenna which has its ground level at the ground of the shielding case conventionally provided within the housing 121 of the mobile or portable radio apparatus 120 and the ground of the circuit board 126 therein. Although the helical antenna 102, being continuously powered, is powered along with the rod antenna 105 when the latter is in its extended position, the helical antenna 102 is then merely operable as an accessory of the rod antenna 105.
On the other hand, when the movable antenna part 104 of assembly 100 is in its retracted position shown on FIG. 1B, there is no electrical connection to the rod antenna 104 and there is a substantial axial spacing between the upper end of rod antenna 105 and the lower end of the helical antenna 102 and the metal members 107, 110 and 122 connected thereto. As a result of the foregoing, electromagnetic coupling between the helical antenna 102 and the rod antenna 105 is avoided when the movable antenna part 104 is in its retracted position. Therefore, rod antenna 105, when in its retracted position, has no affect on the performance of helical antenna 102 which then operates as a single helical antenna having its ground level at the ground of the shielding case inside housing 121 and the ground of the circuit board 126.
In a modification of the antenna assembly according to the prior art which includes a rod antenna 105 and a helical antenna 102, and in which the rod antenna, when retracted into the housing, is electrically isolated from the helical antenna, the stopper 115 of insulating material at the lower end of the movable antenna part 104 is replaced by an electrically conductive metal stopper which is electrically connected with the rod antenna 105 within the cover 106. In such modified example of the prior art, when the movable antenna part is in its extended position, for example, as in FIG. 1A, an electrical circuit for powering the rod antenna 105 is established from power supply circuit 124, through antenna clip 125, mounting member 122, hub 108 of disc 107 and the noted metal stopper that replaces insulating stopper 115, directly to the lower end of rod antenna 105. In this case also, when the antenna assembly is in its extended position, both the helical antenna 102 and the rod antenna 105 are powered and the rod antenna mainly operates as a monopole antenna which has its ground level at the ground of the shielding case conventionally provided within the housing 121 and the ground of the circuit board 126 therein, while the helical antenna 102, being continuously powered once again, is merely operable as an accessory of the rod antenna 105 in the extended position of the latter.
In the retractable antenna assembly according to the prior art which has been described above with reference to FIGS. 1A and 1B, the helical antenna 102 enables the associated portable or mobile radio apparatus, such as, a cellular 'phone, to receive an incoming call even in the retracted position of the rod antenna 105, whereas, with the rod antenna in its extended position, the performance of the antenna assembly is improved for actual communication.
In retractable antenna assemblies of the type described above with reference to FIGS. 1A and 1B, antenna lengths in the extended and retracted positions are designed on the basis of a wavelength of the required frequency band, for example, 1.9 GHz. More specifically, the length of the helical antenna is determined so as to satisfy the required antenna length with the movable antenna part 104 in its retracted position, while the length of the rod antenna 105 is determined to satisfy the required antenna length with the movable antenna part 104 in its extended position. It had been supposed that designing the rod antenna and the helical antenna of the known antenna assembly 100 in the described manner would achieve the optimum antenna performance in the required frequency band. However, at a frequency as high as 1.9 GHz, electromagnetic coupling between the rod or monopole antenna and the helical antenna, which occurs in the extended position of the movable antenna part 104, is likely to produce undesirable resonance. In some cases, such undesirable resonance may be produced around a frequency close to the required frequency band so that the performance of the monopole antenna is degraded thereby.
The undesirable resonance may be moved away from the required frequency band by modifying the parameters of the helical antenna 102, such as, the length, pitch and number of turns thereof. However, such modification of the helical antenna parameters in order to avoid undesirable resonance necessarily changes the helical antenna condition which has been optimized for obtaining the best performance of the antenna assembly 100 in the retracted position of its movable part 104. On the other hand, without such modification of the helical antenna parameters, the resulting undesirable resonance would degrade the performance of the antenna assembly 100 with its movable antenna part 104 in the extended position. In other words, optimizing the performance of the helical antenna 102 with the movable antenna part 104 in its retracted position sometimes conflicts with the avoidance of undesirable resonance in the extended position of antenna part 104.
In an attempt to avoid the foregoing problems, it has been proposed, for example, as disclosed in detail in U.S. patent application Ser. No. 08/729,705, filed Oct. 7, 1996, by Shinichiro Tsuda (one of the inventors named herein) and having a common assignee herewith, to provide an antenna assembly similar to that described above with reference to FIGS. 1A and 1B, but in which the helical antenna includes first and second helical portions which are electrically separated from each other, with one of such helical portions being continuously connected to a power supply, and with the helical portions of the helical antenna being electrically connected with each other, as by a connector mounted on the cover of the rod antenna, only when the rod antenna is in its retracted position at which time the first and second helical portions operate as a single helical antenna.
However, it has been determined that, with an antenna assembly as shown in FIGS. 1A and 1B, or with the antenna assembly described above with reference to the earlier filed U.S. patent application Ser. No. 08/729,705, it is difficult to optimize the impedance characteristics of the antenna assembly so as to provide the same impedance value for both the retracted and extended positions of the rod antenna. In this connection, it is noted that the helical antenna is intended to operate as not only a radiator, but also as a kind of matching circuit for the rod antenna in the extended position of the latter, whereas, in the retracted position of the rod antenna, it is intended that the helical antenna operate only as a radiator. By reason of the foregoing, if the rod or monopole antenna is designed to optimize its radiation characteristic in the extended position, it could be difficult to design the helical antenna to achieve the same impedance when the rod or monopole antenna is in its retracted position. Particular difficulty is experienced in achieving optimized impedance characteristics in both the retracted and extended positions of the rod or monopole antenna at values near to 50 ohms, which is likely to be the impedance value of RF circuits, in the absence of an external matching circuit.